Method and device for measuring the viscosity of a liquid

ABSTRACT

A method and a device are proposed for measuring the viscosity of a liquid ( 12 ), a conclusion being reached on the temperature of the liquid ( 12  at the location of the viscosity sensor ( 20 ), starting from temperature measurements and/or viscosity measurements made in the past.

BACKGROUND INFORMATION

Software-based systems, for indicating that an engine oil change in amotor vehicle is required, are known, of which some are based onalgorithms which evaluate parameters such as, for instance, the mileagecovered since the last oil change or the frequency of cold starts.

Alternatively, other known methods avail themselves of sensor signalswhich describe the physical state of the oil directly, using suitablesensors, for example, the dielectric constant of the oil or, as a farmore reliable quantity, the oil viscosity being measured. In thiscontext, from the determination of the viscosity change of the engineoil since the last oil change, a viscosity-based oil change criterionmay be derived, since engine oil breakdown is usually associated with anincrease in viscosity. In evaluation electronics having a connecteddisplay device in usual systems, for example, a viscosity boundary valuemay be stored, which is compared to measured viscosity values of theengine oil, and when they exceed it, an instruction is given to thedriver as to the next due time for oil change. The physical oilparameters ascertained by these methods known from the related art aretemperature-dependent without exception, so that a temperaturecompensation computation is necessary for the determination. Also, fromunpublished Patent Application DE 100 085 47, a method for theassessment of the breakdown of engine oil is known, in which the engineoil viscosity is measured by a sensor, and in which a measuring sensoris allocated to the sensor for the oil viscosity measurement for thesimultaneous determination of the oil temperature, the oil viscosity andthe oil temperature being measured in the cooling phase of the engine,after it is shut off.

SUMMARY OF THE INVENTION

Compared to this, the method according to the present invention and thedevice according to the present invention, having the features of thealternative independent claims has the advantage that the oil viscositycan be measured at any time, i.e. not only in the cooling phase of theengine. Moreover, the method according to the present invention and thedevice according to the present invention are distinguished by the factthat it is advantageously possible to carry out a more accuratedetermination of the viscosity. For, during the monitoring of liquidoperating substances such as engine oil, a viscosity measurement using aviscosity sensor may be drawn upon for assessing the current liquidstate. However, since viscosity η, as a rule, is a strongly temperaturedependent quantity, it is imperative simultaneously to determinetemperature T by a temperature sensor. A correspondingly ascertainedviscosity-temperature measured value pair {η,T} characterizes a point ofthe viscosity-temperature characteristic curve η=η(T) of the liquid.During the recording of several points at various temperatures, which iseasily possible if the medium to be monitored is heated and cooled inregular operation, as happens, for instance in the case of engine oil,several measured value pairs may be used for interpolation of thecharacteristic curve η(T). The method according to the presentinvention, as given in the main claim has the advantage that theviscosity-temperature measured value pairs {η,T} are determinable withgreater accuracy, and are therefore drawn upon for the more accuratecharacterization of the viscosity-temperature characteristic curveη=η(T), since the viscosity η measured by the viscosity sensor wasgenerally ascertained at a temperature different from the temperaturemeasured by the temperature sensor. The different temperature of theliquid at the location of the viscosity sensor and at the location ofthe temperature sensor, given rise to by the spatial separation of theviscosity sensor and the temperature sensor, comes about due to atemperature gradient in the liquid. Such a temperature gradientpreferably appears when there is a strong heat supply or heat removal inthe liquid to be monitored. Therefore the method according to thepresent invention and the device according to the present invention areadvantageously used when there are large temperature gradients, becausethe viscosity measurements carried out without the method according tothe present invention and the device, according to the presentinvention, in that case would be particularly inaccurate.

The measures specified in the dependent claims permit advantageousfurther refinements and improvements of the method indicated in the mainclaim and the device indicated in the alternative independent claim.

BRIEF DESCRIPTION OF THE DRAWING

An exemplary embodiment of the present invention is represented in thedrawing, and explained in detail in the following description.

FIG. 1 shows a scheme in principle of the measuring setup.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

A container 10 holding a liquid 12 is illustrated in FIG. 1. Incontainer 10, or rather on container 10, a viscosity sensor 20 and atemperature sensor 30 are provided. Both viscosity sensor 20 andtemperature sensor 30 are connected to an evaluation unit 40. With theaid of two arrows denoted by reference numeral 14 it is indicated that,operationally conditioned in container 10 shown in FIG. 1, there comesabout a supply and/or removal of heat and/or liquid 12. Furthermore,liquid 12 in container 10 can be moved, such as by circulating pumps oreven by convection, a temperature gradient being created because ofespecially cold or especially hot locations of container 10.

If it is assumed that there is a temperature gradient in liquid 12 at aspecified time t, then the spatial separation of viscosity sensor 20from temperature sensor 30 has the effect that the temperature of liquid12 deviates at the spatial position of viscosity sensor 20 from that atthe position of temperature sensor 30. According to that, temperatureT₃₀ measured by temperature sensor 30 does not represent temperature T₂₀(unknown, in principle) of liquid 12 at the location of viscosity sensor20.

According to that, viscosity-temperature measured value pairs {η,T₃₀}can a priori not be drawn upon without error to characterize theviscosity-temperature characteristic curve η=η(T), since viscosity ηmeasured by viscosity sensor 20 was generally not ascertained at atemperature different from T₃₀, namely T₂₀. However, the presentinvention describes a method for ascertaining valid measured valuepairs.

Such a temperature gradients preferably appear when there is a strongheat supply or heat removal in liquid 12 which is to be monitored. Thisheat removal or heat supply is shown in FIG. 1 by reference numeral 14and the corresponding arrows. When the heat supply is omitted, thetemperature distribution stabilizes largely by thermal compensationprocedures, so that, within the framework of the attempted accuracy, theequationT₃₀=T₂₀ holds.

According to the present invention it is provided that an algorithm beused which filters out from a series of plotted measured values, ormeasured value pairs, those values which may be used, that havesufficient accuracy for ascertaining the viscosity-temperaturecharacteristic curve η(T). If one starts from a plot of viscositymeasured values η_(k) and temperature measured values T_(30,k) atdiscrete times t_(k), as can be done, for example by a microcontrollerin evaluation unit 40, then values belonging together at a specifiedtime, e.g. at time t_(k) may be combined to a measured value triad

M_(k)={η_(k),T_(30,k),t_(k)}, the index k indicating both at η and atT₃₀ that these measured values were measured at the kth measuring pointin time. In the most general case, the algorithm ascertains, for themeasured value triad which belongs to time t_(k), a dimension numberZ_(k) or a measure Z_(k) which estimates the degree of inhomogeneity inthe temperature distribution on the basis of the current as well as thelast N measured value triad M_(k-1) through M_(k-N), that is, in generalZ _(k) =f(M _(k) ,M _(k-1) , . . . M _(k-N)).

Now, the dimension number or the measure is to be defined in such a waythat the undershooting of a certain threshold value S or a certainthreshold indicates an acceptable reduction in the temperature gradient.In other words: All measured value pairs {η_(k),T_(30,k)} for whoseappertaining dimensionZ_(k)<Sis true, may be regarded with sufficient accuracy as points of theviscosity-temperature characteristic curve η=η(T).

The physical background of the proposed method is the assumption that,because of the thermal compensating procedures, one may draw conclusionswith regard to smaller spatial temperature gradients from temporalchanges in the local temperature that are becoming smaller. As anexample of this, we give an especially simple example for theconstruction of Z_(k). Using the provisionZ _(k) =f(M _(k) ,M _(k-1))=|(T _(30,k) −T _(30,k-1))/(t _(k-1))|the absolute value of the temporal difference quotient of thetemperature measured value is ascertained as a measure of theinhomogeneity of the temperature of the liquid. If measure Z_(k) thusascertained is less than a specified threshold S, the temperaturedistribution is regarded as sufficiently homogeneous. In this example,only the current measured value triad and the measured value triadbefore that in time were considered, that means, N=1 is true.Furthermore, only the temperature measured values were drawn upon forthe construction of Z_(k). Generalizations of this expression, e.g. byevaluation of the change in the measured viscosity value, in order toevaluate the temperature changes at the location of viscosity sensor 20indirectly, and consideration of several value triads lying in the past,are also provided by the present invention.

The present invention also provides that measured value triads becorrected instead of just filtered out. This is carried out withparticular advantage if the heating/cooling mechanism of the liquid isvery well studied and known, and the formation of correspondingtemperature gradients is studied more thoroughly. For example, it isprovided according to the present invention that one may conclude from ameasured temporal temperature change that there was a correspondingerrorε=T ₂₀ −T ₃₀and correct measured value T₃₀ accordingly.

In order to evaluate the temperature gradients, a plurality oftemperature sensors 30 may also be applied spatially separated. In thiscase, instead of the single temperature sensor 30 shown in FIG. 1, aplurality of such temperature sensors are provided at various locations,particularly of container 10, according to the present invention.Thereby the temperature may be estimated at the location of viscositysensor 20, for example, by averaging.

1. A method for measuring a viscosity of a liquid as a function of atemperature of the liquid, the method comprising: providing a viscositysensor at a first location of the liquid; providing at least onetemperature sensor at a second, different location of the liquid;performing at least one of the following: (a) reaching a conclusion onthe temperature of the liquid at the first location of the viscositysensor at a specified point in time, and (b) reaching a conclusion onthe viscosity of the liquid at the second location of the temperaturesensor at a specified point in time; and discarding a measured value ofat least one of the viscosity sensor and the temperature sensor if ameasure of an inhomogeneity of the temperature of the liquid exceeds aspecified threshold, wherein the at least one of (a) and (b) isperformed starting from at least one of the following: (1) at least oneof temperature measurements and viscosity measurements made in the past,and (2) a plurality of temperature measurements at different locationsat the specified point in time.
 2. A method for measuring a viscosity ofa liquid as a function of a temperature of the liquid, the methodcomprising: providing a viscosity sensor at a first location of theliquid; providing at least one temperature sensor at a second, differentlocation of the liquid; performing at least one of the following: (a)reaching a conclusion on the temperature of the liquid at the firstlocation of the viscosity sensor at a specified point in time, and (b)reaching a conclusion on the viscosity of the liquid at the secondlocation of the temperature sensor at a specified point in time; andcorrecting a measured value of at least one of the viscosity sensor andthe temperature sensor as a function of a measure of an inhomogeneity ofthe temperature of the liquid, wherein the at least one of (a) and (b)is performed starting from at least one of the following: (1) at leastone of temperature measurements and viscosity measurements made in thepast, and (2) a plurality of temperature measurements at differentlocations at the specified point in time.
 3. The method according toclaim 1, further comprising drawing upon a temporal difference quotientof at least one of the temperature and the viscosity as the measure ofthe inhomogeneity of the temperature of the liquid.
 4. The methodaccording to claim 2, further comprising drawing upon a temporaldifference quotient of at least one of the temperature and the viscosityas the measure of the inhomogeneity of the temperature of the liquid. 5.The method according to claim 1, further comprising ascertaining themeasure of the inhomogeneity of the temperature of the liquid from atemporal pattern of at least one of the measured temperature and theviscosity of the liquid.
 6. The method according to claim 2, furthercomprising ascertaining the measure of the inhomogeneity of thetemperature of the liquid from a temporal pattern of at least one of themeasured temperature and the viscosity of the liquid.
 7. A device formeasuring a viscosity of a liquid as a function of a temperature of theliquid, the device comprising: a viscosity sensor situated at a firstlocation of the liquid; at least one temperature sensor situated at asecond, different location of the liquid; an arrangement for discardinga measured value of at least one of the viscosity sensor and thetemperature sensor if a measure of an inhomogeneity of the temperatureof the liquid exceeds a specified threshold; and means for performing atleast one of the following: (a) reaching a conclusion on the temperatureof the liquid at the first location of the viscosity sensor at aspecified point in time, and (b) reaching a conclusion on the viscosityof the liquid at the second location of the temperature sensor at aspecified point in time, wherein the at least one of (a) and (b) isperformed starting from at least one of the following: (1) at least oneof temperature measurements and viscosity measurements made in the past,and (2) a plurality of temperature measurements at different locationsat the specified point in time.
 8. The device according to claim 7,wherein a temporal difference quotient of at least one of thetemperature and the viscosity is drawn upon as the measure of theinhomogeneity of the temperature of the liquid.
 9. The method accordingto claim 7, wherein the measure of the inhomogeneity of the temperatureof the liquid is ascertained from a temporal pattern of at least one ofthe measured temperature and the viscosity of the liquid.
 10. A devicefor measuring a viscosity of a liquid as a function of a temperature ofthe liquid, the device comprising: a viscosity sensor situated at afirst location of the liquid; at least one temperature sensor situatedat a second, different location of the liquid; an arrangement forcorrecting a measured value of at least one of the viscosity sensor andthe temperature sensor as a function of a measure of an inhomogeneity ofthe temperature of the liquid; and means for performing at least one ofthe following: (a) reaching a conclusion on the temperature of theliquid at the first location of the viscosity sensor at a specifiedpoint in time, and (b) reaching a conclusion on the viscosity of theliquid at the second location of the temperature sensor at a specifiedpoint in time, wherein the at least one of (a) and (b) is performedstarting from at least one of the following: (1) at least one oftemperature measurements and viscosity measurements made in the past,and (2) a plurality of temperature measurements at different locationsat the specified point in time.
 11. The device according to claim 10,wherein a temporal difference quotient of at least one of thetemperature and the viscosity is drawn upon as the measure of theinhomogeneity of the temperature of the liquid.
 12. The method accordingto claim 10, wherein the measure of the inhomogeneity of the temperatureof the liquid is ascertained from a temporal pattern of at least one ofthe measured temperature and the viscosity of the liquid.